1. Field of the Invention
The present invention relates to space frame systems of the type in which two substantially parallel rectangular grid frames are interconnected by a plurality of diagonal struts.
2. Description of the Prior Art
Space frames are well known in the art, such as the type in which a pair of rectangular grid frames are offset one from the other by half a grid dimension in both directions and diagonal struts run from every grid intersection to the four nearest grid intersections on the opposing frame. Such a geometrical arrangement is disclosed in R. B. Hartmen, U.S. Pat. No. 2,284,898; in C. W. Attwood et al, U.S. Pat. No. 3,821,280; in A. L. Troutner, U.S. Pat. No. 3,466,824; and in H. V. Papayoti, U.S. Pat. No. 3,914,063, and a double thickness three frame arrangement of the same type is shown in H. V. Papayoti, U.S. Pat. No. 3,821,107.
All of these patents excepting Troutner disclose use of generally "U" section chords and struts, which are attached by bolts passing through the bottom of the "U". While this design provides ease of assembly, it has a problem in that it only makes use of a single shear resistance of the bolt. That is, the bolt only has to fail in shear in the single plane between the two bolted-together members in order for the joint to slide apart. Twice the strength in shear is available by use of three-layer designs, the center layer pulled one way and the two outer layers the other, so that the bolt must be sheared apart in two planes to part the joint.
A second and a third problem also come from this one-sided attachment, the second being that it imposes an undesirable bending moment on whatever the member is bolted to, equal to the tensile or compressive force carried by the member times the eccentricity between the attachment plane and the axis of the force. The third problem is that it reduces the maximum force in compression that the member can safely carry. It is usual to design to avoid the failure mode in which the straight member under compression becomes unstable and collapses into a curve, a collapse by either elastic or plastic buckling, and it can be seen that the previously described momemt from the eccentricity starts this off, predisposes this type of curving, and so reduces the safe load.
Troutner discloses "U" chords, but avoids the three problems above with the struts by using for them round tubes, which each end flattened to the center (not eccentric) and bolted in position as the center layer of three layers, in double shear.
Papayoti, in U.S. Pat. No. 3,914,063, discloses use of pairs of "U" section chords or struts, on either side of the connecting member back-to-back at the bottoms of the "U" sections, whereby as to each single member of the pair, all three problems remain, but as to the two members together, there is double shear, and the two bending moments on the joint offset one another. Problem three, the reduced allowable compressive force in elastic or plastic buckling modes, is still a problem, for each member of the pair, and there is no way for this to be compensated or cancelled out.
A fourth problem is that when a "U" section member is placed in compression, the unsupported sides of the "U" may buckle sideways elastically or by plastic deformation, in yet another mode of collapse. Again, Trautner also avoids this problem, at least as to the struts, by using round tubes. Papayoti, in U.S. Pat. No. 3,914,063 discloses a "U" section member with the free edges folded over double, which partially compensates for the lack of support of the sides of the "U".
A fifth problem is the inevitable weakening caused by having a joint in each chord at each grid intersection, as disclosed by all of these patents excepting Hartman. If continuous chords are used and overlapped at grid intersections, as disclosed in Hartman, this problem is avoided, but the chords then no longer lie in a single plane. Eccentricity problems are created by not having all of the chord forces in one plane, not unlike those described for single members previously, with now three instead of two dimensions. Further, if the chords are overlapped, there is more height required for the space frame. It becomes more difficult to mount additional structures, such as skylights, on the uneven overlapped structure. And it becomes more difficult to "stack" one space frame on another, as disclosed, for example, in Papayoti, U.S. Pat. No. 3,821,107.
Many of these problems are reduced or eliminated if all of the members are closed tubes. However, closed tubes are more difficult to interconnect than "U" sections, and particularly so for round sections. Moreover, it is somewhat more difficult to arrange mounting of skylights and other structures upon round section space frame systems. Gugliotta, U.S. Pat. No. 3,882,650 discloses a pipe-and-ball truss array including an interconnection system. There each pipe member is held against connector balls at each of its ends by an internal coaxial rod threaded at the rod ends, extending just beyond the pipe ends through holes in the ball walls, and secured by nuts within. Earlier designs, such as disclosed in British Pat. No. 1,206,399, have threaded pipe ends screwed into corresponding threaded holes in the balls, or have welded the pipe ends to the balls. These systems all tend to be costly.